Add loading skeleton architecture

[anselmbradford]

A "loading skeleton" is a placeholder box before a web component has loaded in.

Changes

• Add loading skeleton architecture.
• Fix issue where tagline was loosing top/bottom padding at mobile screen size.

Testing

1. Find CfpbTagline.init(); and CfpbFormSearch.init(); in index.js and comment them out.
2. yarn build && yarn start
3. Visit http://localhost:4000/design-system/
4. See that there are boxes where each component should be.
5. Undo step 1, and re-run step 2-3.
6. See that the components load in normally. See that there isn't a layout shift when they load in.

[anselmbradford]

I didn't use the SASS mixin here because I wanted to keep this in .css with the idea we'll aim to nix the media-query mixins anyway.